Exemplary embodiments of the present invention relate to a fuel cell system having at least one fuel cell arranged in a housing.
Fuel cell systems having at least one fuel cell are known from the general prior art. The fuel cell is frequently designed as a stack of individual fuel cells, and the fuel cell stack can be used to generate electric drive energy in a vehicle. The fuel cells can be designed as PEM fuel cells.
Such fuel cells or also other types of fuel cells are generally supplied with hydrogen or a hydrogen-containing gas as a fuel for an anode side of the fuel cell and with oxygen or with the oxygen contained in the air on the cathode side of the fuel cell. Various measures are known from the prior art for reacting to potential leaks of such a fuel cell stack and to thereby prevent formation of combustible mixtures of hydrogen or hydrogen-containing gas and air. In this connection the fuel cell stack is typically arranged in a housing. Scavenging air can flow through the housing, as described in, for example, German Patent Document DE 10 2007 042 784 A1. This scavenging air can thereby flush out any slight leakages of hydrogen and/or oxygen from the housing and feed these to an area of the fuel cell system in which either adequate dilution takes place so that an explodable mixture no longer exists, or in which the hydrogen is combusted or catalytically combusted in order to thus avoid hydrogen emissions and ignitable mixtures.
This structure, as described for example in the above-mentioned German Patent Document DE 10 2007 042 784 A1, thereby requires comparatively high resources as various components are needed to maintain a scavenging air flow.
It is also generally known to use flow compressors to the convey air required for the cathode region. Such conveying means for the supply air can, for example, be pure flow compressors that are driven by an electric motor. Structures are also known and usual, however, in which single-stage or multi-stage conveying means are formed, of which at least one stage is formed in the manner of a turbocharger. Furthermore, so-called electric turbochargers are known and usual. These use, in a turbine arranged on a common shaft with a flow compressor, residual energy from the region of the fuel cell system or optionally thermally generated energy, for example from a (catalytic) burner in order to thereby drive the flow compressor. At the same time an electric machine is provided on the shaft, which works—according to the power needed and supply—either as an electric motor in order to support the flow compressor or as a generator if there is more power on the turbine than is required for the flow compressor. Any contamination of this supply air flow, for example with lubricating oil or similar, could lead to lasting damage to the fuel cell and must therefore be avoided. In order to ensure that in such structures no lubricating oil contamination or similar is caused for the cathode region of the fuel cell, it is common to use air bearing systems in order to thus be able to achieve a very pure supply air flow to the cathode region of the fuel cell.
Exemplary embodiments of the present invention to involve a fuel cell system that avoids the abovementioned disadvantages and guarantees a highly functional structure with a simple, compact and cost-effective fuel cell system.
In accordance with exemplary embodiments of the present invention the partial air flow, which flows through at least a part of the conveying means as bearing air and/or cooling air, flows in the flow direction before or after the conveying means as scavenging air through the housing of the at least one fuel cell. In the structure according to the invention the bearing air and/or cooling air that is branched off from the supply air to the fuel cell after the conveying means is used for the conveying means in order to flow through, besides at least one part of the conveying means as scavenging air, also the housing of the at least one fuel cell. Any leakages and/or ignitable or explosive mixtures of any gases leaving the fuel cell are thereby taken up in the known way and can be discharged to the environment diluted by the scavenging air or are supplied for other uses and/or post-combustion to suitable components. Thus, the present invention maintains full functionality without requiring an additional fan or similar for the scavenging air of the housing. This avoids the need for a conveying means and the associated costs as well as the construction space required by this conveying means. The fuel cell system according to the invention can therefore be designed to be more compact, lighter and more cost-effective than the fuel cell systems according to the prior art.
According to a further particularly favorable embodiment of the fuel cell system according to the invention the scavenging air flows through the housing in the flow direction after the conveying means. This flow direction ensures a comparatively high pressure in the region of the conveying means so that the bearing air and/or cooling air arrives here securely and reliably with the required pressure and the required volume flow. After flowing through the conveying means and the pressure losses unavoidably arising, the scavenging air then arrives in the region of the housing. Since no high pressure losses typically arise and no through-flow with a particularly high volume flow is required, the remaining flow energy of the scavenging air after flowing through the conveying means is completely adequate so that no energy or only minimal energy must be expended by the conveying means in order to realize the additional flushing of the housing with the scavenging air. The structure thus saves construction space and components without energy expenditure or with only minimal energy expenditure in comparison with the conventional structure. In relation to a structure with its own conveying means for the scavenging air, not only construction space and construction volume are thereby saved but also energy.
According to a very advantageous development of the system according to the invention the scavenging air can also flow through at least one heat exchanger between the conveying means and the housing. The temperature in the region of the scavenging air before entering the housing can thereby be correspondingly reduced in order to avoid risk of heating the housing and the at least one fuel cell located therein through the scavenging air. This is particularly advantageous if the scavenging air had also assumed cooling tasks in the region of the conveying means, for example cooling of an electric motor drive for the conveying means. A gaseous or liquid cooling medium can thereby flow through the heat exchanger in order to thus cool the scavenging air. The gaseous cooling medium could, for example, be the cooling medium present in a cooling circuit of the fuel cell system.
According to an alternative or supplementary embodiment of the invention a line element is also provided for the scavenging air between the conveying means and the housing, which is formed to be longer than would be necessary for the pure flow connection. By means of a line element designed to be longer and which preferably extends in cooler regions of the fuel cell system, it is possible alternatively or additionally to the above-mentioned heat exchanger for a cooling of the scavenging air to be achieved before entry into the housing through the longer flow path.
According to a favorable development of the fuel cell system according to the invention the scavenging air flows, after flowing through the housing, into a region which comprises catalytically active material.
The feeding of the scavenging air into a region with catalytically active material is very suitable for removing any hydrogen emissions produced through leakages of the at least one fuel cell within the housing. Emissions to the environment are thereby avoided and it is possible to securely and reliably avoid an ignitable mixture that constitutes a potential risk. The region with the catalytically active material can thereby have a different configuration. It is conceivable, for example, as known from the prior art, to use a recombinator that merely serves to use up any hydrogen contained in the scavenging air or to convert it with air oxygen to water. Alternatively or additionally, it is possible to feed the scavenging air, after through-flow of the housing, for example, by the supply air, to the cathode region. Since the electro-catalyst of the at least one fuel cell is present as a catalytically active material in the cathode region, it is possible to react the minimal quantities of hydrogen transported here with the scavenging air without negatively influencing the performance of the fuel cell. As this catalyst must be present anyway in order to maintain the electrochemical functionality of the fuel cell, it is possible with this structure to omit a catalyst provided specifically for reacting the scavenging air. There is thus a clear cost advantage when using, for example, platinum as a catalyst due to the comparatively high costs associated with the catalyst. In addition there is an advantage in relation to the construction space and the weight. Alternatively, however, a catalytic burner can be used which is designed, for example, so that exhaust gases from the anode region and the cathode region combust therein and the hot exhaust gases drive an expander that is combined, for example, with the conveying means, as already described above. In such a structure the scavenging air can then also be supplied to this catalytic burner. In this way it is possible to ensure, without requiring an additional catalytically active material, that no hydrogen emissions reach the environment with the scavenging air.
According to an advantageous embodiment of the fuel cell system according to the invention the scavenging air can flow in the flow direction before and/or after the housing and/or the conveying means through further components to be cooled and/or heated. The scavenging air will correspondingly heat up upon flowing through the conveying means as bearing air or cooling air and should normally arrive without too high a temperature in the region of the housing. It can therefore be used previously, for example, to heat components to be heated. The cooling air branched off anyway as a partial flow or supply air can be used, alternatively or additionally hereto, also for cooling further components.
In particular, in the case of a cold start of the fuel cell system, the scavenging air, which typically heats as bearing air and/or cooling air of the conveying means, can be used without intermediately arranged or active heat exchanger or other components to be heated, to heat the fuel cell stack within its housing. This allows the cold start time for the fuel cell system to be correspondingly shortened without having to provide additional energy for this. After a successful cold start it is then possible, for example through an activation through flow-through of the abovementioned optional heat exchanger with a cooling medium, for example a gaseous or a liquid cooling medium, for a cooling of the scavenging air to take place before entry into the housing in order to thus ensure that no unnecessary heat enters the region of the fuel cell stack during normal operation.
According to an advantageous further development of the fuel cell system according to the invention the conveying means can also comprise an electric machine and an expander. This embodiment of the conveying means is also described as an electric turbocharger and is known in itself from the prior art. The expander thereby uses energy present in the exhaust gases of the fuel cell in order to thus reduce the drive power for the conveying means brought via the electric machine as a motor. In case of a high power on the expander and low consumption of the conveying means the electric machine can also be operated as a generator in order to convert the energy recovered through the expander into electrical energy.
According to a further very favorable embodiment the expander can expand exhaust gas or at least a fuel cell, wherein the exhaust gases flow through a burner in the flow direction before the expander. In such a burner, which can be formed, for example, as a pore burner or in a preferred development as a catalytic burner, the exhaust gases of the fuel cell can be burnt. These contain, in any case, residual oxygen in the exhaust air from the cathode. Furthermore, depending upon the structure and type of the fuel cell, at least a residual amount of hydrogen is contained in the exhaust gases, for example if the anode exhaust gas is fed in the circuit around the anode. From time-to-time a release of the gas in the circuit is necessary in order to avoid—through the enrichment of inert gases—the hydrogen concentration in the anode region falling below a certain value. In this case hydrogen is contained in the released gas that can be combusted together with the residual oxygen in the air. According to an alternative structure of the anode region without a circuit, hydrogen is typically supplied in a certain excess quantity in the anode region in order to achieve an even flow to the available active surface. In this case the anode exhaust gas then continuously contains a certain residual quantity of hydrogen that can be combusted with the residual oxygen in the cathode exhaust air. As already mentioned above, the scavenging air can also be fed after the housing for this combustion. Any hydrogen contained therein is thus also combusted. As a result of the combustion, hot exhaust gases are produced that output more energy upon expansion in the expander than if the overpressure in the exhaust gases is used to recover energy.
According to a further very favorable embodiment an optional fuel can also be fed to the burner. This structure allows, by means of optional fuel, for example hydrogen or an initial substance for the production of hydrogen, which is carried along in the fuel cell system, to optionally heat the burner. In this way a comparatively high thermal energy can be provided in the region of the expander. The power thus obtained via the expander can then be converted in the electric machine into electrical power that can be used, for example, to cover short-term power peaks, as may arise in particular during acceleration of a vehicle equipped with such a fuel cell system. By means of the expander and the burner a boost function can thus be realized through the optional fuel.